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what compression should i be running?

Discussion in 'The Garage' started by bigoltrux77, Nov 16, 2002.

  1. bigoltrux77

    bigoltrux77 1/2 ton status

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    i have a modified 350 small block. It has a mild cam, and a few other mods, nothing special. tomorrow im going to buy a compression gauge so that i can adjust my valves and get all the right compression that i want. Someone told me that i should run a compression of 9:1. Does that sound right? I want the best performance for my engine. Another thing is i dont really know how to use a compression gauge to do this. Can somoeone tell me the steps taken for this project? Im planning on doing this on Sunday.
     
  2. reddog64

    reddog64 1/2 ton status

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    you cant "test" for compression in the way of 9.0 to 1

    You are going to get a compression tester that will tell you your cyl holds 140 = of pressure before the rings leak and/or valve's leak...

    The 9.0 to 1 compression is a computed value that takes your block, heads, pistons, bore, stroke, and other things into play...
     
  3. willyswanter

    willyswanter 1/2 ton status GMOTM Winner

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    Yeah, what he said, your compression is pre-determined by what pistons you installed and the shape and size of the cylinder and head volume. Only thing a compression gauge tells you is how many psi are in the chamber on the compression stroke.
     
  4. ratlover

    ratlover 1/2 ton status

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    There are 2 kinds of things you can measure with a pressure gauge.

    1 is leak down like the guys are saying. You pump air into the cylinder and see how much air pressure is lost to see the condition of your motor. It can show you if valves are leking or the rings. You can also compare each cylinder to see if you have a bad one.

    2 is looking at your cranking compression, wich is kinda what you are talking about. There are 2 kinds of compression ratios. Static(the kind they are talking about, a measure of the pison, head, and head gasket) and dynamic. Dynamic is the compression ratio that the motor "sees" and is what is important. It takes into account the static compression and then adds to the equation the valve timing(your cam). By runing measuring your "cranking compression" you basicly see what your dynamic compression is but in terms of PSI instead of a ratio. You just hook up a guage into the sparkplug hole and crank your motor over. Do it with the other plugs yanked. You can see if a cam lobe is tosted by looking for differences between cylinders.

    You wont be able to "change" your compression ratio without new parts......well you can change your dynamic/cranking but not much. You can alter it a bit by altering your cam timing. You can do this with messing with your lash(only applies to solid lifters) or by advancing or retarding your cam. This involves a bit of work for a motor that is all put together and you wont see much difference.

    By looking at your cranking compression we will be able to see if you have any badly mismatched parts and there are ways to fix that but it will involve rasing your compression with new heads. Or a cam swap might be in order.

    Post your cranking compression #'s and we can go from there.

    Are you happy with its performance, just looking for a bit more, or is it just a slug?
     

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